As industry develops, glass or metal conventionally used in various industry fields has been limited in extensive application due to its intrinsic properties. By way of example, ceramic materials such as glass have the problem of being easily broken with external shock or bending, and metals also have the disadvantages of low transparency and high specific gravity. In recent years, the conventionally used glasses or metals have been gradually substituted by polymer materials due to the above-described problems. However, these polymer materials have a lower surface hardness (i.e., pencil hardness) than glass and thus have the disadvantage of being easily scratched by friction. Therefore, with an increase in demand for the polymer materials, a hard-coating technique for improving the polymer materials has attracted a lot of attention.
Korean Patent Laid-open Publication No. 10-2010-0111671 suggests a resin composition for forming a hard coated layer, formed of a (meth)acrylic copolymer obtained by copolymerizing a vinyl group-containing monomer including a quaternary ammonium group and a (meth)acrylic monomer copolymerizable with the vinyl group-containing monomer, a polyurethane oligomer including three or more functional vinyl groups, and/or an acrylic monomer including two to six functional vinyl groups, and a film obtained by coating and curing the resin composition on a substrate. However, such a film has a pencil hardness of from about 4 H to about 6 H (JIS K 5600-5-4), and, thus, it is not suitable for application to the fields requiring a high hardness. Further, the cured product based on the acrylate has the disadvantage of a high contraction ratio than a cured product of an alicyclic epoxy group.
Further, U.S. Pat. No. 8,110,296B2 suggests a crystalline hard coating including a metastable mixed crystal formed of a carbide or nitride of a transition metal such as titanium with a siloxane oxide- or zirconium oxide-based ceramic. However, such a hard coating requires a vacuum deposition-based process such as PVD (Physical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition) instead of a resin-based process, and, thus, it is very inefficient in terms of cost and time. Further, most of the prior art techniques relating to hard coating focus on providing a high hardness and a high scratch resistance, but when such properties are applied to a film, its flexibility decreases. Thus, when a film is formed or bent, a hard coated layer is broken, which makes it difficult to achieve commercialization. Actually, it is difficult to produce a transparent hard coated film which includes a scratch-resistant surface having a pencil hardness of from 3 H to 9 H and is flexibly bent. Therefore, in order to expand the application of the film, it is necessary to develop a hard coating technique capable of maintaining a flexibility of a substrate film and also achieving a high surface hardness.